As billions of people around the world climb out of poverty, their need for basic raw materials will require increasing mining activity. Seeking a social license to operate, “leading mining companies have recently formulated, and pledged to follow, standards and principles for sustainable development of mineral resources worldwide” (chapter 16.1, page 1644, “Site Environmental Considerations” by Michael G. Nelson in the SME Mining Engineering Handbook, 2012, the contents of which are incorporated by reference herein in their entirety). To earn such license from the community, mine operators must consider the mine's legacy after closure: How will the environmental legacy look? Will the social and economic legacy be the “boom and bust” of historic mining communities? There is significant need for a mine legacy that is environmentally, economically, and socially beneficial.
The need for low cost sustainable energy is well known. Fortunately there are many sources of economical or even free low-grade thermal energy, such as industrial waste heat, hot summer air, or cold winter air. A challenge is their intermittent production. There is a strong need for economical storage of low-grade thermal energy, so that it can be used whenever needed.
The present disclosure is directed to addressing these needs with a thermal energy reservoir which is a legacy product from a mining system. The reservoir may be constructed in situ from mined waste rock at small cost incremental to ordinary mining activity. It stores low-grade thermal energy when economically available, and supplies thermal energy as needed. For example, free hot summer air can be stored for use in winter as needed.
Most alternative energy endeavors require massive up-front capital investment, governmental incentives, and long payback periods. By focusing on low-grade thermal energy and leveraging mining activity which is proceeding anyway, a small incremental cost enables creation of a large-scale alternative energy system with short payback. More than just environmentally friendly, the legacy reservoir is a valuable asset that gives an on-going vibrant community a competitive edge by supplying sustainable energy at cost well below conventional sources.
Note: As used here, “thermal energy” means a capacity to heat or cool some object to a desired temperature. “Low-grade” means that temperature differences are relatively small, typically less than 100 degrees Celsius, in contrast to “high-grade” thermal energy such as the steam in a power plant which may be at 500 degrees.